While it has been known that in a CZ silicon wafer there are crystal defects referred to as so called Grown-in detects such as COP (Crystal Originated Particle) and an oxide precipitate, heat treatment performed in hydrogen atmosphere (hereinafter also referred to as hydrogen annealing) has been proposed as a method for annihilating Grown-in defects in the vicinity of a wafer surface. This heat treatment requires use of hydrogen at a high temperature of 1000° C. or higher; therefore, a safety measure is required, and an ordinary open furnace (for example, a furnace unsealed on the furnace opening side such as a horizontal furnace) cannot be used for the treatment, so that it is necessary to provide a sealing structure for enhancing air-tightness and an explosion-proof facility as a countermeasure against explosion, resulting in very high cost.
On the other hand, it has been recently found that the Grow-in defects can be annihilated in heat treatment carried out in argon atmosphere (hereinafter also referred to as Ar annealing) as in the hydrogen annealing. Since the Ar annealing has no explosiveness, a safer operation is ensured as compared with the hydrogen annealing, but it has also been known that the Ar annealing displays a characteristic behavior to a silicon wafer in contrast to the safety operation. There is given as an example of the characteristic behavior a fact that tiny pits are easily formed on a surface of a wafer subjected to the Ar annealing.
This pit formation is described as follows: An oxide film is formed by oxygen and water as very small amounts of impurities included in raw material gas or oxygen and water in the open air involved through the opening of a reaction tube during a heat treatment step or when unloading a wafer, and the oxide film further reacts with silicon (Si) according to a reaction of SiO2+Si→2SiO; consequently Si is etched, the etched sites being observed as pits. The pits contribute to degradation of local surface roughness (micro-roughness) and long periodic surface roughness (haze) on a wafer surface. Thus, Ar gas is sensitive to a trace of impurities, and small environmental changes such as temperature fluctuations, so it has the demerit of difficulty in handling.
There are proposed the following methods for preventing such degradation of surface roughness of a wafer in the Ar annealing: one is to reduce a water concentration in raw material gas and the other is to form an etching-resistant film by treating the wafer in oxygen or nitrogen atmosphere prior to unloading it from a heat treatment furnace after the Ar annealing (JP A 93-299413).
As described in the above published patent application, however, while a nitride film is produced in nitrogen treatment at 1000° C. or higher, the nitride film produced at 1000° C. or higher has a very slow speed in HF etching, compared with an ordinary natural oxide film, and is not easily etched in cleaning with SC1 (a mixture of NH4OH/H2O2/H2O), SC2 (a mixture of HCl/H2O2/H2O) or the like; therefore, it affects heat treatment in the next step. In addition, since a nitride film has a dielectric constant higher than an oxide film, in other word, a higher electrostatic capacitance, charged particles are easy to attach thereto and particles that have been once attached thereto are hard to be removed. This is a drawback when a nitride film is grown on a wafer surface.
On the other hand, tiny pits are also easily generated when switching Ar gas to oxygen gas prior to unloading a wafer after the Ar annealing. This is because, if oxygen of a prescribed amount or higher is not present in Ar gas (in other words, an oxygen concentration of a prescribed amount or lower), a region where an oxide film is formed is etched by the following reaction:SiO2+Si→2SiO.This phenomenon inevitably occurs as a transitional one when switching Ar to oxygen. Such etching occurs when a partial pressure of oxygen is a prescribed value or lower even if oxygen is introduced after Ar is removed by a vacuum pump.